Assert constructs ConstantExpression with null when no comment is given

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Description

I ran into this problem with Lipstick after deploying ASSERT. When ASSERT is not given any comment, LogicalPlanBuilder constructs ConstantExpression with null. Later tools like Lipstick try to convert it to a String resulting in NPE.

Although it's possible to handle this case in Lipstick, I feel ConstantExpression shouldn't be constructed with null in the first place.